The present invention generally relates to ink-jet printing, and in particular to a specific ink formulation for photo printing. This ink formulation provides for improved ink-jet print quality.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area-fills, and other patterns thereon. Low cost and high quality of the hardcopy output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality. A surge in interest in ink-jet printing especially in the area of photographic printing has resulted in the need to produce high quality prints at a reasonable cost. The challenge remains to further improve the print quality of ink-jet prints. The emerging use of ink-jet prints for digital photos, requires high-resolution images that have accurate color, are durable, and do not show banding of colors.
Color ink-jet printers, such as a DesignJet(copyright) printer available from Hewlett-Packard Company, typically use three inks of differing hues: magenta, yellow, and cyan, and optionally black. The particular set of colorants, e.g., dyes, used to make the inks is called a xe2x80x9cprimary dye set.xe2x80x9d A spectrum of colors, e.g., secondary colors, can be generated using different combinations of the primary dye set. In printing processes such as lithography, dye transfer, and some types of thermal transfer, it is possible to vary spot size so that less is demanded of the primary colored inks when producing a good secondary color. When the color inks are used in a binary printing device, that is, one in which a dot of color is either present or absent, e.g., a thermal ink-jet printer, the ability of the primaries to give recognizable secondary colors is even more important. When such device is to be used for printing images that will also be printed and compared to images printed by other modalities such as offset presses or dye sublimation printers, it is important that the colors produced by the inkjet printer are capable of encompassing and matching commonly accepted industry color descriptions, such as the color gamut (color space) for a Kodak Duralife(copyright) silver halide print. If such ink is to be used in an ink-jet printing device, characteristics such as crusting, long-term stability, and materials compatibility must also be addressed. If the ink is to be used in a thermal ink-jet printer, the further constraint of being thermally stable (kogation-resistant) is added.
In general, a successful ink set for color ink-jet printing must be compatible with the ink-jet pen and the printing system. Some of the required properties for the ink-jet ink include: good crusting resistance, good stability, the proper viscosity, the proper surface tension, little color-to-color bleed, rapid dry time, no deleterious reaction with the printhead components, high solubility of the dyes in the vehicle, consumer safety, low strike through, high color saturation, good dot size which affects the banding, and suitable color characteristics.
Inks are known which possess one or more of the foregoing properties. However, few inks are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations. For example, inks are known which can obtain the color gamut of silver halide prints, such as those used obtained in photographic prints, but cannot obtain the smooth appearance of non-banding of such silver halide prints. Accordingly, investigations continue into developing ink formulations which have improved properties and which do not improve one property at the expense of the others. Thus, there remains a need in the art to further improve the print quality, color gamut, and banding properties of the ink-jet prints without sacrificing pen performance and reliability, particularly when trying to reproduce the color gamut of silver halide prints.
In accordance with the invention, inks suitable for use in ink-jet inks and method for formulating the same are provided. It has been found that a synergy can be produced by the combination of certain branched alcohols or diols with 1,2-diols containing a polar head group and a non-polar region. This combination provides a basis for an ink formulation that provides for good photo print attributes, especially good dot size which results in a non-banded appearance of the printed image, with improved reliability. Alcohols and diols have often been added to ink vehicles as humectants to reduce crusting in inks. However, humectants often increase the viscosity of the ink and hurt the dot size of the resultant ink. The synergy present between the selected alcohols or diols and the 1,2-diols results in higher reliability and increased dot size, a powerful combination. This formulation works best with glossy, coated paper mediums as compared to plain paper mediums. Further, this combination allows for a surfactant-free formulation. In fact, a preferred embodiment herein is an ink formulation substantially free of surfactants or other organic or inorganic solvents besides the alcohol or diol/1,2-diol combination.
In the practice of this invention, yellow, cyan, and magenta aqueous inks each comprise from about 0.1 to about 20 wt % of at least one colorant in the color ink formulations; with black ink comprising from about 1 to about 20 wt % of at least one black colorant in the black ink formulations; from about 0.5 to about 2.0 wt % of at least one mono- or di-hydric alcohol, and from about 7 to about 20 wt % of at least one 1,2-diol having a suitable hydrophile-lipophile balance (HLB). Additionally other independently selected ingredients can be added including those from the group consisting of buffers, biocides, and metal chelators; and the balance water.
The present ink compositions offer good dot size which results in a non-banded appearance of the printed image and are reliable in an ink-jet printing engine.